Can I block the IAP purchases?

Yes, you can block the IAP directly from the game. To do that go to Options > Personal Settings > Disable In-App Purchase.

In order to block IAP purchases on Mobile Devices, we recommend the Parents/Guardians to follow these steps given by Apple and Google respectively.

Apple IAP Block: https://support.apple.com/en-us/HT204396

Google IAP Block: https://support.google.com/googleplay/answer/1626831?hl=en

In order to prevent purchases via PayPal we recommend to withhold from saving the PayPal credentials on PC.